In computer-assisted operations, the position and orientation of surgical instruments with reference to the patient's anatomical structures are represented to the surgeon with the aid of a navigation system (e.g., on the navigation system's display device). In addition to the anatomical structures, the instruments used in the surgical procedure also are trackable using suitable reference systems, which enable precise representation of the instrument. This usually is achieved via active or passive marker structures that disclose their position in the field of operation to the navigation system by emitting or reflecting infrared radiation.
In a registration process, the anatomical structures are initially correlated with reference systems mounted on them via navigable pointers, and are thus spatially trackable by the navigation system in the subsequent operation process. The same is required for the instruments and their reference systems, wherein the instrument functional elements are particularly important. These are surfaces of tools (e.g., cutting edges, tips, etc.) which, for example, can be used to treat bone structures, wherein the surgeon employs a navigation system to obtain precise information concerning the position and orientation of the bone structures, despite possible impaired visibility. Representing the spatial position of the instrument and its functional elements on the display device is based on a correlation of stored geometric data of the instrument, and the spatial data can be ascertained by the navigation system via the reference system attached to the instrument. As soon as the stored geometric data, which mainly describe the functional elements in relation to the reference system, deviate from the actual geometry of the instrument (e.g., after the instrument has been damaged), this inevitably leads to an incorrect representation of the relation between the actual instrument and the anatomical structure to be treated (if the information stored in the database concerning the initial outline of the instrument continues to be used).
Currently, the relation between the functional element of the instrument and its reference system is established once on the basis of manufacturing documents, wherein their compliance is ensured by surveying after the manufacturing process. The relation is stored in the navigation system and is retrieved and used for subsequent operations. These so-called pre-calibrated instruments (e.g., bore guides) are occasionally verified pre-operatively using navigable aids to confirm that they are dimensionally accurate. If they are not sufficiently accurate, this is displayed to the user who should then avoid using the instrument (assuming a calibration as described below is not possible). Matching the model data to the actual existing, possibly deviating instrument is not possible in verification. The precision which verification can achieve is inevitably dependent on the precision of the navigation system, since the respective reference systems are again correlated with each other. Visibility problems and occasionally poor manageability when simultaneously positioning the instrument and the aid tend to extend the operation time.
In another method, a calibration procedure of the instrument is carried out before the instrument is used. The values of functionally relevant parameters of the instrument (length, diameter, etc.) required for navigation but still unset, or unset values for assigning the reference system to the functional element (e.g., socket driver with variable sockets) or the deviations of the actual instrument from the model stored in the database are determined by means of a navigable calibration tool, temporarily or permanently saved in the navigation system and retrieved as needed. Here, too, the visibility and manageability problems described above have a negative effect on the operation time. Calibration functions well for instruments having functional elements with simple geometries, but again only within the limits of the precision which the navigation system can achieve.
Completely surveying functionally relevant structures of an instrument that is difficult to calibrate or verify represents a third method which, however, is not currently employed, since suitable aids are not available, nor could the procedures required be carried out by the operating staff within a justifiable time frame.